1. Technical Field
The present invention relates to a projection system suitable to be incorporated into a projector that enlarges and projects an image formed on an image display device, and also relates to a projection-type image display apparatus into which the projection system is incorporated.
2. Related Art
In recent years, as the luminance and resolution of a projector are increased, a projector can be installed at a wider range of locations, and there is an increasing demand for installation of a projector at a location relatively apart from a screen. To meet the demand, it is necessary to provide a long-focus zoom lens that not only performs projection over a relatively long distance from the screen to meet a specific screen size but also achieves high resolution.
That is, as the luminance of a projector is increased, the projector is increasingly used in an application in which the projector itself is installed at a location apart from a screen. A zoom lens having a long focal length and a large f-number and providing high resolution is required as a projection lens accordingly.
As a typical projection system, there is a retro-focus projection system in which a first group closest to a screen has negative power. Using such a projection system to form a long-focus system undesirably results in a large diameter of the entrance pupil. To achieve a large diameter of the entrance pupil, the diameters of the lenses in a second group and the following groups undesirably increase due to on-axis light rays. As a result, it is difficult to ensure high performance. Further, the diameter of a mechanism in a projector body increases and the diameters of the lenses increase, resulting in an increase in cost.
On the other hand, there are long-focus zoom lenses used as an imaging system (see JP-A-2007-157097, JP-A-2009-237400, and JP-A-2008-164997).
For example, using the imaging system described in JP-A-2007-157097 as a projection system increases the size of a front lens, the overall length of the projection system, and the diameter of an aspheric lens, resulting in a difficulty in manufacturing, an increase in cost, and other problems. To avoid the problems, optical system design adapted to a projection application is required.